What's a good action for my Tbird? I had it high and low but I get horrible tone on any frets 9 and higher

Good action is highly personal and subjective. It's essentially defined as what works best for you. I have my Epiphone Thunderbird Classic set at 2.25 mm (E string) and 1.75 mm (G string) bottom of string to top of fret - BUT - I use smaller gauge strings than you, so you will probably want something higher.

Also sounds like you may not have your truss rod adjusted to give the correct neck bow. So that would be the first thing to check and correct.

To adjust your truss rod, you need a capo, a 0.015 inch feeler guage, and the appropriate wrench or screw driver to fit your truss rod.Also, you can break the truss rod and/or ruin your neck if you do this badly. You are warned.

To proceed:

Once you've got the neck adjusted correctly, try setting your low pitch string (B in your case, I think) 1/8 inch above the top of your 12th fret by adjusting your bridge height. (You can't adjust individual saddle height on the three post Gibson/Epiphone bridge.) You will mostly be adjusting the rear-most screw on the low string side, but you might also do a little adjusting of the front middle screw.

Then the same sort of thing on the high pitch string side (D for you, I think). You want to aim for just under 1/8 inch - a tiny bit lower than the B string.

Once you've got that, go through each string along the entire fretboard checking for fret buzz. If you've got fret buzz, your truss rod isn't adjusted to give the correct bow, or your frets or neck is fucked up. If the neck is bowed correctly and not messed up, you will have to raise the action to lift the strings higher to escape the fret buzz.

If you don't have buzz, raise or lower the action to your liking. Commonly, low action is sought after. Too low and you will get fret buzz, then you have raise them back up a bit until no buzz.

I like to lower my action on each string until I hear the buzz, the I raise each until the buzz cuts off then I know it's to my liking. I like it to be super low so I can produce notes with a heavy punch with my string hand (right), with using minimal effort so I don't strain my fret hand (left). It is nice to have effortless action. With your truss rod, you can take an Allen wrench and fit in the end and turn the rod, make sure you take the neck off the body first though. You want to look down the frets and see if there is a bow like shape towards the sky or ceeling, if so, then the rod is good. The strings will make the neck completely straight because they pull on it when they are tuned up.

Hey Marko, have you ever built your own bass? Not bolt pre-fab stuff together, but took a block of wood and shaped and made your own bass?

I've always wanted to do this. I'm somewhat crafty with wood (I build furniture and stuff for my kids), so I'm confident I could build the body. But I'm sure I'd mess up the neck and would have to buy a bolt on neck.

IamMark yes I have, I made the body from solid ash, I'll admit I bought the neck, a maple Jazz Bass job with block markers, and cut the headstock to my own shape. It's got two Stingray Humbuckers and an active pre-amp and it sounds awesome. I'm in the process of sourcing the materials for my second bass but this time I'll be making the neck aswell. It's gonna have a mahogany body with cavities routed in to reduce weight and a quilt maple top, the headless neck will be set into the body, also mahogany with ebony board and 35" scale, the bridge is a Steinberger D B model with drop D and B facility it will have 3 EMGs, a Jazz each at the neck and middle and a Stingray at the bridge with EMGs active electronics